I agree with on Humes' meaning with the is/ought divide. I would not say that Hume was merely pointing out some logical error or syllogistic fault. But to your credit Hedo you acknowledged this possibility in your later post. In any case, the three of us should agree that contemporary philosophical understanding interprets Humes is/ought divide in this more general sense that Wayfarer speaks of. So it's what we must contend with.

I like the angle you are approaching this from. The conclusion is not novel, but the way you reach it is. Most with your predilections don't acknowledge the uncertainty aspect, or if they do, they handwave it as a foolish thing to be concerned with.

Now, this Agrippa trilemma that you bring up fascinates me, and I'm glad you brought it up. IMO, this is the core problem of ethics. My opinion on this is that the first sort of justification is a "moral" one. I say this because before empiricism or any "ism," for that matter, gets off the ground; you must take some evaluative stance. I could elaborate further if anyone wants me to.

As I understand it, this is the path most moral realists take. They say something like, before you can even make the first step to philosophy, you must tacitly value—in a "moral" sense—the inclination to the truth. I think there's a case to be made that before you can even be skeptical, this inclination to truth must be present. But, not too sure of that, and I am willing to be convinced otherwise.

Any thoughts on this?

I'm sure it's already been touched on in the replies, and I'm far too lazy to check, but utilitarianism is arbitrary, but not for the reasons you point out. The obvious reply to this sensical objection is to qualify that it is the foreseeable consequences which matter. Of course, we cannot predict every foreseeable effect an action might have. It is too impractical.

We only need to concern ourselves with the reasonably foreseeable consequences. For example, if Billy going into the woods with a lighter on leads, somehow to the end of world hunger, we can safely disqualify this as a predictable consequence. However, if Billy starts a ravaging fire that displaces several animals and kills many more, well, that is slightly more predictable. It is a bit ironic that you should charge consequentialism as being unpredictable and arbitrary, though.

Think for a moment of how utterly awful we are at ascertaining or predicting our true intentions. Think about how many things are deeply layered in our subconscious.

There is also this point to make. Intentionalism does not matter when one can claim ignorance to reasonably predictable consequences. For example, let's say that it was not Billy's direct intention to kill and displace all those animals that day. Even if this is true, one might rightly say that it must have been clear to Billy that there was at least some chance the whole forest could burn, and Billy was ok with this chance. It is obviously unwise to sanction an ethical theory that does not make individuals responsible for predictable results.